The present invention relates to the art of blow-molding freestanding plastic drums, and more specifically to the drum bungholes. Conventionally, plastic drums are manufactured by extruding a cylindrical parison into a chamber defined by open mold sections, inserting at least one blow pin into one end of the parison, closing the mold sections which compresses a portion of the parison against the blow pin forming the bunghole, blowing air through the blow pin and into the parison so as to expand the remainder of the parison to the interior surface of the closed mold, opening the mold, and then removing the blow pin from the final blown article.
In such a method, the blow pin generally has an annular shoulder which forms a sealing surface on the molded bunghole for abutting a reciprocal surface on the bung, or closure. Additionally, the blow pin assembly generally would include means for providing the bunghole with threads for cooperative engagement with a closure. Such means might consist of threads on the blow pin itself for casting the image onto the molding plastic, or it might consist of a threaded plastic or metal insert temporarily affixed to the blow pin but over which the plastic may be molded and which remains as a part of the bunghole upon removal of the blow pin.
One of the problems associated with the sealing surfaces of bungholes made by the above method is that the surfaces generally slope upwardly and outwardly from the center axis of the bunghole, thereby requiring the use of a highly elastic and compressible gasket to effect a seal.
One of the shortcomings of the compression molded plastic threads has been their poor definition leading to cross-threading and reduction of their mechanical advantage. Finally, as will be pointed out below, a related inefficiency of the above-described method of manufacturing plastic drums, with respect to compression molding over a threaded plastic insert, is the difficulty of bonding the adjacent surfaces of the molding plastic and the insert without considerable preheating of the insert.